Just one cigarette a day carries substantial risk of heart disease and stroke

Allan Hackshaw and colleagues just published “Low cigarette consumption and risk of coronary heart disease and stroke: meta-analysis of 141 cohort studies in 55 study reports” in BMJ  that blows away the idea that e-cigarette users who cut down the number of cigarettes they smoke without quitting entirely will have health benefits, especially for heart disease and stroke.

Here is the BMJ press release on the paper, which sums it up nicely:

Just one cigarette a day carries much greater risk of heart disease and stroke than expected, warn experts

No safe level of smoking exists; smokers should aim to quit instead of cutting down

Smoking just one cigarette a day has a much higher risk of developing coronary heart disease and stroke than expected - about half the risk of smoking 20 per day - concludes a review of the evidence published by The BMJ today.

The researchers say their findings have important consequences for many smokers and health professionals who believe that smoking only a few cigarettes carries little or no harm. They argue that smokers should stop completely instead of cutting down to significantly reduce their risk of heart disease and stroke. This information might be particularly helpful at the start of a new year, when many smokers are likely to be thinking about stopping or smoking less.

Individual studies have reported that smoking only one to five cigarettes per day is associated with a higher than expected risk of heart disease. To investigate this further, a team of researchers led by Professor Allan Hackshaw at the UCL Cancer Institute at University College London analysed the results of 141 studies and estimated the relative risks for smoking one, five, or 20 cigarettes per day.

They found that men who smoked one cigarette per day had 46% of the excess risk of heart disease and 41% of the excess risk of stroke associated with smoking 20 cigarettes per day (much higher than the expected 5%).

For women, those who smoked one cigarette per day had 31% of the excess risk of heart disease and 34% of the excess risk of stroke associated with smoking 20 cigarettes per day. Women’s heart disease risk was more than doubled with one cigarette per day, when only studies that controlled for several factors were included in the analysis. 

 “We have shown that a large proportion of the risk of coronary heart disease and stroke comes from smoking only a couple of cigarettes each day,” say the authors. “This probably comes as a surprise to many people. But there are also biological mechanisms that help explain the unexpectedly high risk associated with a low level of smoking.”

They acknowledge some study limitations, but say their paper is the first to combine results across many studies covering both coronary heart disease and stroke, “making it a valuable reference that can be used to strengthen public health campaigns, and provide a strong health incentive for smokers to stop completely (particularly women).”

Cardiovascular disease, not cancer, is the greatest mortality risk for smoking, causing about 48% of smoking-related premature deaths. 

The authors conclude: “No safe level of smoking exists for cardiovascular disease. Smokers should quit instead of cutting down, using appropriate cessation aids if needed, to significantly reduce their risk of these two common major disorders.”

In a linked editorialKenneth Johnson,  Adjunct Professor at the University of Ottawa outlines the major public health implications of these results, and says “only complete cessation is protective and should be emphasised by all prevention measures and policies.”

The take home message for smokers is that “any exposure to cigarette smoke is too much,” he adds. “The message for regulators dealing with newly marketed 'reduced risk' products is that any suggestion of seriously reduced coronary heart disease and stroke from using these products is premature.”

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Here is the abstract:

OBJECTIVE:  To use the relation between cigarette consumption and cardiovascular disease to quantify the risk of coronary heart disease and stroke for light smoking (one to five cigarettes/day).

DESIGN: Systematic review and meta-analysis.

DATA SOURCES:  Medline 1946 to May 2015, with manual searches of references.

ELIGIBILITY CRITERIA FOR SELECTING STUDIES: Prospective cohort studies with at least 50 events, reporting hazard ratios or relative risks (both hereafter referred to as relative risk) compared with never smokers or age specific incidence in relation to risk of coronary heart disease or stroke.

DATA EXTRACTION/SYNTHESIS:  MOOSE guidelines were followed. For each study, the relative risk was estimated for smoking one, five, or 20 cigarettes per day by using regression modelling between risk and cigarette consumption. Relative risks were adjusted for at least age and often additional confounders. The main measure was the excess relative risk for smoking one cigarette per day (RR_{1_per_day}-1) expressed as a proportion of that for smoking 20 cigarettes per day (RR_{20_per_day}-1), expected to be about 5% assuming a linear relation between risk and consumption (as seen with lung cancer). The relative risks for one, five, and 20 cigarettes per day were also pooled across all studies in a random effects meta-analysis. Separate analyses were done for each combination of sex and disorder.

RESULTS: The meta-analysis included 55 publications containing 141 cohort studies. Among men, the pooled relative risk for coronary heart disease was 1.48 for smoking one cigarette per day and 2.04 for 20 cigarettes per day, using all studies, but 1.74 and 2.27 among studies in which the relative risk had been adjusted for multiple confounders. Among women, the pooled relative risks were 1.57 and 2.84 for one and 20 cigarettes per day (or 2.19 and 3.95 using relative risks adjusted for multiple factors). Men who smoked one cigarette per day had 46% of the excess relative risk for smoking 20 cigarettes per day (53% using relative risks adjusted for multiple factors), and women had 31% of the excess risk (38% using relative risks adjusted for multiple factors). For stroke, the pooled relative risks for men were 1.25 and 1.64 for smoking one or 20 cigarettes per day (1.30 and 1.56 using relative risks adjusted for multiple factors). In women, the pooled relative risks were 1.31 and 2.16 for smoking one or 20 cigarettes per day (1.46 and 2.42 using relative risks adjusted for multiple factors). The excess risk for stroke associated with one cigarette per day (in relation to 20 cigarettes per day) was 41% for men and 34% for women (or 64% and 36% using relative risks adjusted for multiple factors). Relative risks were generally higher among women than men.

CONCLUSIONS:  Smoking only about one cigarette per day carries a risk of developing coronary heart disease and stroke much greater than expected: around half that for people who smoke 20 per day. No safe level of smoking exists for cardiovascular disease. Smokers should aim to quit instead of cutting down to significantly reduce their risk of these two common major disorders.

The full citation is:  Low cigarette consumption and risk of coronary heart disease and stroke: meta-analysis of 141 cohort studies in 55 study reports. Hackshaw A, Morris JK, Boniface S, Tang JL, Milenković D. BMJ. 2018 Jan 24;360:j5855. doi: 10.1136/bmj.j5855.

The accompanying editorial by Ken Johnson, “Just one cigarette a day seriously elevates cardiovascular risk” makes the point that

“Finally, e-cigarettes and heat-not-burn products should not be promoted for “harm reduction” on the grounds that they lead people to smoke fewer cigarettes, because modest reductions in cigarette consumption are unlikely to have meaningful health benefits and dual use of cigarettes and e-cigarettes may expose smokers to increased total risks. Furthermore, e-cigarettes are reducing smoking cessation rates, and marketing of supposedly safer tobacco products seems to recruit and addict new generations of young smokers.

“The take home message for smokers is that any exposure to cigarette smoke is too much. The message for regulators dealing with newly marketed “reduced risk” products is that any suggestion of seriously reduced CHD and stroke from using these products is premature [citations omitted].”

The citation for the editorial is: Just one cigarette a day seriously elevates cardiovascular risk.  BMJ 2018; 360 doi: https://doi.org/10.1136/bmj.k167 (Published 24 January 2018) Cite this as: BMJ 2018;360:k167